1. Field of the Invention
The present invention relates to a brake system of a brake-by-wire type vehicle, and more particularly, to technology of adaptively braking a vehicle in response to fails in a control unit or fails in electronic brakes respectively installed at the wheels, in a brake system that includes the electronic brakes installed at the wheels and achieves braking by an electronic control unit.
Further, the present invention relates to a technology of preparing for fails in the brake system of brake-by-wire type vehicles, that is, a fail-safe technology of a brake system.
2. Description of the Related Art
In general, the brake-by-wire is a kind of new technology used for vehicles in recent years, which has been developed to remove several parts and simplify assembly by using electronic control called ‘by-wire’. That is, it is a concept used for controlling braking force by using electronic signals, replacing hydraulic brakes of the related art, in which the brake pedal unit and the brake actuator, such as the wheel cylinder, are separated.
The brake-by-wire brake system is composed of an operation pedal unit, a brake controller, and a brake actuator. The operation pedal unit is similar to the pedal units of the hydraulic brakes of the related art; however, electronic signals are generated from a common pedal stroke sensor, instead of signals that are generated by hydraulic pressure from a master cylinder and a booster. The brake controller is an electronic control circuit, which receives signals from the operation pedal unit and outputs operation signals by calculating appropriate brake pressure from pedal-pressing force and a stroke etc. The brake actuator is a wheel cylinder, a caliper of a disc brake, or an electronic motor, and applies braking force to the wheels of a vehicle in response to the operation signals from the brake controller.
The brake-by-wire brake system (hereafter, referred to as an electronic brake) and a hydraulic brake system of the related art are briefly compared hereafter.
A hydraulic brake system strongly pushes a pad to a disc in braking, using hydraulic pressure and has a configuration including not only the hydraulic line connected to a master cylinder operated to generate hydraulic pressure by a booster increasing pedal operation force and a wheel cylinder, but various devices controlling and complimenting the cylinders, such that the configuration is correspondingly complicated. Accordingly, there are some limits in ensuring safety and reliability of braking performance using hydraulic pressure, due to the complicated configuration.
On the other hand, an electronic brake system has a simple configuration, which the hydraulic brake system does not have, and can improve reliability of braking performance and implement a parking brake function. Further, it can improve response and performance of an ABS (Anti Brake System) and implement optimal global chassis control. In particular, a study has been conducted in recent years, which uses an EWB (Electro Wedge Brake) system that implements braking by pressing a brake pad against a disc to generate friction in braking, using a wedge assembly operated by an actuator, in order to compensate small braking force of common EMBs (Electro Mechanical Brake). The EWB can achieve braking force, which the hydraulic brake system can generate, with only a motor using 12 (Volt) voltage, because the EWB implements self-energizing using wedge action. In other words, large braking force can be achieved, even if force from the motor is small, by using a wedge structure in which as a wedge is moved by operating the actuator, the pad is pressed and friction force between the pad and the disc acts as additional input force.
Further, the EWB has the advantage of automatically compensating wear of the pad when the pad is worn, that is, providing a function of adjusting a gap due to the worn pad by moving the wedge assembly to the pad.
However, in spite of many advantages, the electronic brake system is pointed out that a vehicle with wheels equipped with the electronic brakes has considerably lower safety than a vehicle equipped with the hydraulic brakes. This depends on whether the electronic brake system can brake the vehicle under unexpected situations, such as a fail or an error in the operation of the control unit, because the electronic brake system has a basic structure that calculates braking force by using the ECU (Electronic Control Unit) and supplies power to the signal line and the driving motor that drives the actuator.
Accordingly, methods for dealing with fails of an electronic brake used as a braking device of a vehicle have been proposed. This is described with reference to FIG. 1.
FIG. 1 is a diagram illustrating a brake system of the related art.
As shown in FIG. 1, a brake system 1 of the related art includes a pedal unit 300, electronic brakes 110, 120, 210, and 220 mounted at the wheels, respectively, WCUs (Wheel Control Unit) 11, 12, 21, and 22 controlling the electronic brakes, a first ECU 100, and a second ECU 200.
The brake system 1 of the related art receives brake signals (usually electronic signals) transmitted from the pedal unit 300, using the first and second ECU 100 and 200, respectively, at the center portion of the vehicle. The reason that the first and second ECUs 100 and 200 are doubly equipped is for implementing emergency braking with the other ECU, when a fail occurs in any one of the two ECUs. Meanwhile, the first ECU 100 transmits control signals to the WCUs 11 and 22 controlling the first electronic brake 110 and the fourth electronic brake 220 respectively, in which a signal connection line is formed in so called X-split structure to increase safety in emergency braking.
In the brake system of the related art, however, for example, when a fail simultaneously occurs in the WCU 11 controlling the first electronic brake 110 and the second control unit 200, braking is consequently performed only by the fourth electronic brake 220 at the rear wheel, which causes the driver to be exposed to significant danger.
As a result, since the brake system of the related art is equipped with two central control units (ECU) and four WCUs, it has a problem that the manufacturing cost increases. Further, there is another problem in that it is difficult to efficiently brake a vehicle, because the electronic brakes that the WCUs and the ECUs are in charge of, respectively, are fixed even in the brake system having the double structure.